Flash vacuum pyrolysis

Flash vacuum pyrolysis (FVP) is a technique in organic synthesis. It entails heating a precursor molecule intensely and briefly. Two key parameters are the temperature and duration (or residence time), which are adjusted to optimize yield, conversion, and avoidance of intractable products.[1] Often the experiment entails volatilizing a precursor, which is drawn through a "hot zone" followed by rapid condensation. The apparatus typically is conducted under dynamic vacuum. The hot zone must impart heat to the gaseous molecules, so it is generally packed with solids to induce gas-solid collisions. The packing material is generally chemically inert, such as quartz.[2] The precursor (i) volatilizes with gentle heating and under vacuum, (ii) the precursor fragments or rearranges in the hot zone, and finally (iii) the products are collected by rapid cooling. Rapid post-reaction cooling and the dilution inherent in gases both suppress bimolecular degradation pathways.

Examples

The technique is applied to conversions that proceed via unimolecular pathways. 2-Acetoxydioxane, when heated at 425 °C converts to the highly reactive dioxene, via loss of acetic acid. [3] 2-Furonitrile has been prepared by flash-dehydration of 2-furoic acid amide or oxime over molecular sieves.[4] The strained ring benzocyclobutenone has been prepared by FVP from a simple benzoyl chloride precursor.[5]

Illustration of FVP process, the intermediate ketene is unstable and the product, like most organic compounds, tolerates brief, but not prolonged treatment at high temperatures.[5]
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References
  1. McNab, Hamish "Chemistry without reagents: synthetic applications of flash vacuum pyrolysis" Aldrichimica Acta 2004, volume 37, pp. 19–26. http://www.sigmaaldrich.com/ifb/acta/v37/acta-vol37-2004.html#20
  2. Wentrup, Curt "Flash (Vacuum) Pyrolysis Apparatus and Methods" Australian Journal of Chemistry 2014, volume 67, 1150–1165. doi:10.1071/CH14096
  3. Matthew M. Kreilein, James C. Eppich, Leo A. Paquette "1,4-Dioxene" Org. Synth. 2005, volume 82, pp. 99. doi:10.15227/orgsyn.082.0099
  4. Jacqueline A. Campbell; McDougald, Graham; McNab, Hamish; Rees, Lovat; Tyas, Richard (2007). "Laboratory-scale synthesis of nitriles by catalyzed dehydration of amides and oximes under flash vacuum pyrolysis (FVP) conditions". Synthesis. 20 (20): 3179–3184. doi:10.1055/s-2007-990782.
  5. Peter Schiess, Ppatibha V. Barve, Franz E. Dussy, Andreas Pfiffner (1995). "Benzocyclobutenone By Flash Vacuum Pyrolysis". Org. Synth. 72: 116. doi:10.15227/orgsyn.072.0116.CS1 maint: uses authors parameter (link)
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